Fuel cell power systems convert a fuel and an oxidant into electricity. One such fuel cell power system has a proton exchange membrane (hereinafter also referred to as “PEM”) to catalytically facilitate the reaction of fuels (such as hydrogen) and oxidants (such as oxygen or air) into electricity. The PEM is a solid polymer electrolyte that facilitates transfer of protons from the anode to the cathode in each individual fuel cell of the stack of fuel cells present in a fuel cell power system.
In a typical fuel cell assembly, or stack, each fuel cell has flow fields in flow communication with manifolds that provide channels for the various reactant gases to flow into each cell. Gas diffusion assemblies then distribute the reactants from the flow fields to the reactive anode and cathode of a membrane electrode assembly (hereinafter also referred to “MEA”).
Effective operation of a PEM fuel cell requires proper humidification of the PEM to maintain its proton conductivity. At the same time, the flow field channels and gas diffusion assemblies must be maintained in non-flooded operational states. In operation, the oxidant is supplied to the cathode where it reacts with hydrogen cations that have crossed the PEM and electrons from an external circuit. The fuel cell generates both electricity and water through the electrochemical reaction. The water is typically removed with the cathode effluent, which may dehydrate the PEM unless the water is otherwise replaced. It should be noted that the rate of evaporation to the cathode is generally greater than the rate of water generation.
When hydrated, the polymeric PEM possesses “acidic” properties that provide a medium for conducting protons from the anode to the cathode of the fuel cell. However, if the PEM is not sufficiently hydrated, the “acidic” character diminishes, and may impede the desired electrochemical reaction of the cell. Hydration of the PEM also assists in temperature control within the fuel cell, insofar as the heat capacity of water provides a heat sink. In addition to issues of water balance and cell hydration, another issue in fuel cell design is the efficient use of space. For example, space in a vehicle is precious and designs that minimize the ongoing use of space in the vehicle clearly benefit the utility of the vehicle; this leads toward integration of the humidifying system into each of the fuel cells.
The need for efficiency in operation and greater integration of cooling and humidification to achieve efficient space utilization in fuel cell systems continues to be strongly felt. What is needed is a fuel cell power system which provides integrated humidification of the feed gases (especially the oxidant) and cooling of the MEA.